Fluid pressure responsive capacitive transducers comprising a variable capacitor mounted in a fluid chamber having a thin ceramic diaphragm exposed to the fluid so that a change in fluid pressure causes concomitant changes in the position of the diaphragm to thereby cause change in the capacitance of the capacitor are well known in the art. Transducers of this type are shown and described, for example, in U.S. Pat. No. 4,716,492, the subject matter of which is incorporated herein by this reference. In that patent, a thin ceramic diaphragm is shown mounted in closely spaced, sealed, overlying relation to a ceramic substrate. Metal coatings are deposited on opposing surfaces of the diaphragm and substrate to serve as capacitor plates arranged in predetermined closely spaced relation to each other to form a capacitor. Capacitor terminal pins having one end connected to the capacitor plates are mounted in bores extending through the substrate with opposite ends connected to signal conditioning circuitry disposed in an electric circuit chamber at an opposite face surface of the substrate. A connector body of electrical insulating material, received over the signal conditioning circuitry, mounts transducer terminals extending into the electric circuit chamber for connection to the signal conditioning circuitry. Capacitance is converted by the circuitry to an output voltage related to the fluid pressure. The capacitor is received in a suitable housing having a fluid receiving port and is provided with a fluid seal enabling the transducer to be used with fluid pressures up to 10,000 psi or higher.
In certain applications it is desirable to measure temperature of fluid as well as pressure. A combined pressure responsive transducer and temperature sensor is shown and described in U.S. Pat. No. 5,974,893, the subject matter of which is incorporated herein by this reference. In that patent a variable capacitor having a rigid substrate and a flexible diaphragm, each provided with a capacitor plate on a respective face surface, are attached to each other in aligned, overlying, spaced apart relation by a generally annular glass seal. The variable capacitor is disposed in a housing with the diaphragm exposed to fluid in a fluid pressure chamber of the housing that is in fluid communication with a fluid pressure receiving port of the housing. Electrical traces extend from the capacitor plates into an enclosed window in the glass seal for electrical connection with electrical pins extending through the substrate. A temperature responsive element is mounted for direct engagement with electrical leads that extend through an opening in the diaphragm near its outer periphery aligned with the glass seal.
Temperature sensor pins extend through the substrate and are connected to respective connection pads. The other ends of the temperature sensor pins, as well as the electrical pins for the variable capacitor, are connected to signal conditioning circuitry provided in an electric circuit chamber formed between the housing and a connector body attached to the housing. In one embodiment a temperature responsive element such as a thick film thermistor is coated onto the exposed face of the diaphragm along with electrically conductive traces connected to the connection pads. Other embodiments have the thermistor disposed within a fluid receiving port of the housing or disposed beyond the port with or without a protective sheath.
Each of the above embodiments of the U.S. Pat. No. 5,974,893 patent have certain limitations. With respect to the embodiment having the thermistor coated onto the exposed face of the diaphragm, it is desirable to reduce the response time of the temperature sensor and to provide a temperature sensor that has enhanced environmental compatibility, that is, a sensor that has greater immunity to the corrosivity of the media to which it is exposed. With regard to mounting the thermistor so that it is disposed within the fluid receiving port or at a location beyond the port, it is desired to provide a combined fluid pressure and temperature responsive sensor that is less costly to manufacture, one that is easier to assemble as well as one that is more robust while still having a short response time.